There is a wide variety of known techniques for propelling a marine vessel. Marine vessel herein is meant to refer to ships, boats, watercraft, and other vessels operating in fresh or saltwater, and is not restricted to ocean vessels. Manual techniques for propelling marine vessels include the use of oars, paddles, and poles. Sails also provide propulsion without the need of motors. However, motorized propulsion typically provides greater control and greater speed.
Motorized marine propulsion techniques include the use of paddle wheels, screw propellers, and water jets. Paddle wheels are uncommon except in nostalgic riverboats and lake paddle-steamers, e.g. stern-wheelers and side-wheelers, since conventional paddle wheels are bulky and tend to be inefficient. The paddle wheels are basically “pushers” in which flat paddle planks are rotated through water, thereby using the viscous flow resistance of the paddle to propel the marine vessel along the surface of the water. The inefficiency results from the insertion and extraction losses, as well as turbulence losses. In comparison, the screw propeller exhibits turbulence losses, but is somewhat more efficient because the propeller remains submerged. Water jets direct a high speed stream of water from a nozzle. While water jets provide advantages over other techniques, inefficiency results from the high levels of wetted surface and turbulence involved in moving an incompressible fluid through an often complex configuration at high velocity.
In general, water manipulation in propulsion systems for marine vessels is very lossy, especially when the water is tightly constrained and/or takes on a negative pressure equal to the vapor pressure of the water. This last effect is called cavitation and is very destructive. Propellers create huge turbulence as water is forced to flow around various surfaces, and vortexes abound. A propeller vortex is a spinning water column where the core is a vapor hole or vacuum. This takes a lot of energy to form as it contains huge viscosity losses. When the core collapse, it blows erosion pits into the steel hull and rudder assembly of the marine vessel, incurring extensive maintenance and repair costs. Ducted waterjet pumps are even more lossy, as they have all the pumping losses associated with the associated ducted enclosure and the viscosity of water. Water is difficult to duct at high velocity as it is non-compressible, dense and viscous.
While the known techniques operate well for their intended purposes, further advantages are sought. Such advances may be in one or more of a number of areas, such as efficiency, speed, safety, and adaptability.